How does any sin warrant hell?

Can someone please help me understand the relationship between wrong doing and the need for hell as a result? (sin and its wages which are death).
I feel like I am not as contrite as I need to be when in confession because I don’t understand the gravity of my sins. A lot of people fool themselves into thinking they’re going to heaven because “I’ve never killed anyone or cheated on my spouse, and I’m a good person,” but I know that’s not taking things seriously enough. I just don’t understand why venial sins merit eternal separation from god. Why couldn’t there just be purgatory and not hell? Please explain or point me to some reasons that are more helpful than the catechism on how God is so offended by sin, why he’s offended by sin, and why it merits hell. Thank you.

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